A Template is a piece of sheet material previously formed to a shape that might otherwise be difficult to reproduce accurately. It might be something that you buy from a supplier, or you might be given a full sized or scaled down drawing to transfer to sheet, cut out and sand to the required accuracy. Generally speaking the more complex the finished the component, the greater the benefit of using a template to produce it.
As an example you might be working on a table with 4 legs that are not parallel sided. By making a template for the leg profile, you can use the template to ensure that each leg profile is identical to the others. Yes you spend some time and use some material making the template and getting the edges perfect etc, but you do it once on a thin piece of material, not 4 times on 3inch thick material.
Once you have a satisfactory template you can sit it onto your component material, and trace around it with a pencil. You can use the outline as a guide to bandsaw the shape required, allowing a small amount of excess material beyond the line. Then by using a router with a flush trim (bearing at tip) or pattern (bearing on shank) bit, and fixing the template to the component blank, you can route the component to final shape (and close to final finish) by working around the component with the bearing running against the template and ensuring that you cannot route beyond the desired edge.
As an alternative to a bearing bit, you could fix a template guide to the base of your router. The router bit will pass through the hollow centre of the guide. With this system, there will be some offset between the edge of the template and the path of the router bit, and this is normally factored into the template design. In use, the template guide follows the template in the same way as the bearing on a pattern (bearing on shank) bit would, however the bit is generally free to be plunged and withdrawn as required. This is often not possible with a pattern bit as once the bit moves up or down a moderate amount, the bearing would leave the template edge, allowing loss of control with potential to damage the component and the template. Possible applications include routing complex shapes,plunged surface decoration, mortices etc. One possible template guide application where plunge and withdraw would not be appropriate would be routing dovetails where changing the bit depth would change the fit of the joint.
Hope this helps.
Last edited by malb; 04-26-2013 at 04:53 AM.